2 Answers

Shaymin-S can really be a pain in the ass if used correctly. If you predict a switch use either growth or thunder wave depending on which one you chose. Air slash has a 60% Flinch rate which is sweet and Speed flare can knock down the opponent's Sp. Def. Earth Power is mainly there for steel and Rock Types. Dragon types will be a problem admittedly, but I assume that you're not gonna mess around with waiting for Shaymin to get good IV's to get Hidden Power [Ice]

Thats very good but im pretty sure return is physical and synthesis would probally show it as a stalling pokemon ,so if your using synthisis and using shaymin as a staller I would switch return fortoxic or leech seed, but if it is more of a sweeper I would switch synthesis for psychic(coverage and actually pretty strong) and switch return for either earth power or mud slap( coverage pretty strong and welll pretty awesoeme if you have good special attack).now if you only intend for sunny day to help shaymin and none of youre other pokemon I would switch sunny day for enerrgy ball and if you plan on having a stat up move I would switch out solarbeam for psych up and if not switch it for substitute(if you have a stat up move you can load up and get ready for a ffinal blow and if you don't have a stat move you get atleast one free attack with out getting hurt)